The present invention relates to communications equipment, and, more particularly, to wireless transmission.
Generally speaking, a communications system comprises a transmitter, a receiver, and a communications channel. The transmitter encodes information for transmission and transmits a signal (the "transmitted signal") over the communications channel to the receiver. Typically, the communications channel distorts the transmitted signal to a degree such that the signal at the receiver (the "received signal") is different from the transmitted signal. As such, one of the functions of the receiver is to process the received signal to mitigate the effects of channel-induced distortion.
One type of channel-induced distortion is known as "frequency offset." While a contributor to frequency offset is any mismatch between the transmitter and receiver local oscillators, or clocks, frequency offset is more apparent in a mobile communications channel because of Doppler effects. The latter are caused by the fact that the transmitter and receiver are not stationary with respect to each other. In the context of a communications system in which the encoded information is transmitted via "transmitted signal points," or "symbols," a frequency offset rotates the symbols in the signal space. As a result, at the receiver, each received signal point is offset from its expected position in the signal space.
One method for compensating for frequency offset in a receiver makes use of the well-known "slicing operation," in which the receiver compares values of each received signal point to the known symbol values and selects the closest symbol as an estimate of the transmitted symbol. For example, consider a mobile communications system in which transmission occurs between a transmitter and a receiver for a relatively long period of time, referred to herein as a "continuous transmission" system. In such a transmission system, it is known in the art to design the receiver to spend some initial time period at the start of reception in which the receiver adapts to any frequency offset. During this initial time period, the receiver assumes that any difference in values between a received signal point and the closest symbol is due to a frequency offset. The receiver then computes an average frequency offset estimate over this initial time period and uses this estimate to rotate the received signal point in the opposite direction. Since the transmission period is long, and there is continuous transmission from the same transmitter (or source) to the same receiver (or destination), this initial time period to acquire the frequency offset estimate incurs negligible overhead.
In contrast, in a "bursty transmission" system the effect of frequency offset on receiver design is more severe. In a bursty transmission system, such as, e.g., a mobile time-division-multiple-access (TDMA) system, transmission between a transmitter and a corresponding receiver occurs in TDMA bursts of relatively short duration. In addition, the receiver is typically receiving, in an interleaved fashion, TDMA bursts from different transmitters, where each TDMA burst suffers from a different amount of frequency offset. As such, the receiver must, somehow, quickly estimate for each TDMA burst the respective frequency offset.
In combating the effect of frequency offset in a TDMA-type bursty transmission system, those in the art have applied an approach similar to the above-mentioned acquisition method used in a continuous transmission system. In particular, each TDMA burst, or frame, uses a predefined "synchronization word" (sync-word) as illustrated in FIG. 1. The receiver compares the received sync-word to the predefined sync-word to generate a single estimate, or sample, of the corresponding frequency offset (described further below).
Unfortunately, in a mobile communications channel reflections of the transmitted signal also occur. As a result, the receiver picks up not only the transmitted signal but additional ghosts of the transmitted signal, or what is referred to as "delay spread." Although the above-mentioned approaches are useful in combating the effects of frequency offset by itself, they are limited in their ability to effectively estimate frequency offset in the presence of significant delay spread.